'Seaforth Highlanders watch through a periscope as their trench mortars fire a salvo', 1915 (c)

Photograph, World War One, Western Front (1914-1918), 1915 (c).

A mortar is essentially a short tube designed to fire a projectile at a steep angle so that it falls straight down on the enemy. Such weapons were ideally suited for trench warfare, especially where the lines of the combatants were very close together. As this image demonstrates, mortars could also be fired from the relative safety of the trench.

From a collection of 101 stereoscopic photographs entitled 'The Great War'.